In electron beam devices such as CRTs, the preferred electron beam cross section is not always rotationally symmetric so as to produce a circular spot on the display screen. For example, in recent years color CRT electron gun designers have adopted various asymmetric lenses in their designs to improve the overall performance of the raster display. In these asymmetric lenses, a rotationally symmetric electron beam can give rise to undesired aberration due to mismatched electron lens and electron beam shapes. The designer's goal is to provide an electron beam with a desired cross sectional shape which does not produce undesired electron beam aberration.
An example of electron beam aberration caused by mismatched electron lens and electron beam shapes can be explained with reference to the sectional view of a prior art QPF electron gun 10 shown in FIG. 1. Electron gun 10 is intended for use in a color CRT and thus includes three inline cathodes 12a, 12b and 12c. Electron gun 10 further includes a beam forming region (BFR) 58 comprised of a G.sub.1 control electrode, a G.sub.2 screen electrode, and the low voltage side of a G.sub.3 electrode. Electron gun 10 further includes a symmetric prefocus lens 60 comprised of the high voltage side of the G.sub.2 electrode, a G.sub.4 electrode and the low voltage side of a G.sub.5 electrode. The three electron beams are focused on a display screen of a CRT (which are not shown in FIG. 1 for simplicity) by means of a main focus lens comprised of the high voltage side of the G.sub.5 electrode and a G.sub.6 electrode. A sectional view of electron gun 10 shown in FIG. 1 taken along site line 2--2 therein illustrating the high voltage side of the G.sub.5 electrode is shown in FIG. 2. The G.sub.1 electrode is typically maintained at zero voltage, while the G.sub.2 and G.sub.4 electrodes are typically coupled to a common V.sub.G2 voltage source and the G.sub.3 and G.sub.5 electrodes are coupled to a common focus voltage V.sub.F source. The G.sub.6 electrode is typically coupled to an accelerating, or anode, voltage V.sub.A source. Each of the three electron beams is directed through a plurality of aligned apertures in the various electrodes of electron gun 10 as the electrons proceed from cathodes 12a, 12b and 12c toward the CRT's display screen.
More specifically with respect to the electron gun's main focus lens 64, the low voltage side of the G.sub.5 electrode includes spaced apertures 30a, 30b and 30c aligned with inner apertures 34a, 34b and 34c for passing respective electron beams. The high voltage side of the G.sub.5 electrode includes a peripheral wall 66 defining an elongated, recessed portion 32 which functions as a common lens for the three electron beams. The facing low voltage side of the G.sub.6 electrode includes an elongated, recessed portion 36 also forming a common lens for the three electron beams. The high voltage side of the G.sub.6 electrode includes three spaced apertures 38a, 38b and 38c for passing respective electron beams toward the CRT's display screen.
Referring to FIG. 3, there is shown a sectional view of the electron gun 10 shown in FIG. 1 illustrating only the G.sub.2, G.sub.5 and G.sub.6 electrodes of the electron gun for simplicity, it being understood that the remaining electrodes shown in FIG. 1 are also included in the electron gun shown partially in FIG. 3. In FIG. 3, line A--A' represents the red electron gun axis, line B--B' represents the green electron gun axis, and line C--C' represents the blue electron gun axis. As shown in FIG. 3, the three electron beams respectively transit apertures 16a, 16b and 16c in the G.sub.2 electrode prior to passing through the main focus lens comprised of the G.sub.5 and G.sub.6 electrodes. The main focus lens 64 applies an asymmetric electrostatic field to the three electron beams. This asymmetric electrostatic field arises from the inline alignment of the three electron beams and the shape of the common lens portions of the G.sub.5 and G.sub.6 electrodes formed from facing recessed portions 32 and 36. The facing common lens portions of the G.sub.5 and G.sub.6 electrodes form a combined optimum tube and yoke (COTY) lens.
The effect of this asymmetrical electrostatic field and resulting forces applied to the outer electron beams as they transit the common lens portion of the G.sub.5 electrode is shown in the upper portion of FIG. 3. It can be seen that electron beam rays crossover the axis of each of the electron guns prior to being incident upon a phosphor coating 42 deposited on an inner surface of the CRT's display screen 40. This electron ray crossover is effected primarily by the main focus lens 64 of the electron gun 10. The two outer electron beams form left and right beam spots 52 and 54, while the center electron beam forms a center beam spot 50 on phosphor coating 42. As shown in FIG. 3, the outer electron beam rays in the two outer electron beams undergo a greater focusing effect by the main focus lens 64 than the inner rays (those rays disposed closer to the axis B--B' of the center electron gun). Outer electron beam rays in each of these outer beams are deflected a distance r.sub.1 after crossover, while inner rays are deflected a distance r.sub.0 from the respective center axes A--A' and C--C' of the two outer electron guns, where r.sub.1 &gt;r.sub.0. The increased inner deflection of the outer rays in the two outer electron beams arises from the asymmetric electrostatic field applied to the electron beams.
The result of the application of this asymmetrical electrostatic focusing field on the two outer electron beams is more clearly shown in FIG. 3a which is a sectional view taken along site line 3a--3a in FIG. 3. The axis of the center electron beam is shown as element 44, while the axes of the left and right outer electron beams are respectively shown as elements 46 and 48. From the figure, it can be seen that the over-focusing of the outer rays in the two outer electron beams prior to crossover gives rise to asymmetrical outer electron beam spots 52 and 54. Outer electron beam spot 52 includes an inward directed extension 52a caused by over-focusing of the outer rays as the electron beam transits the main focus lens of the electron gun. Similarly, the right electron beam spot 54 includes an inwardly directed extension 54a also caused by over-focusing of the outer rays as the electron beam transits the main focus lens. Inward extensions 52a and 54a, which are sometimes referred to as a beam spot tail or flare, appear as spherical aberration on the CRT's display screen and degrade video image quality. This electron beam spot tail also appears in the center beam spot when the center beam is deflected off-axis.
An obvious approach to correcting for this electron beam spherical aberration is to intercept the beam with a properly shaped aperture in an electrode of the gun. However, mechanically intercepting the electron beam by means of a physical obstruction in the beam path gives rise to other problems. For example, an electrostatic field in the region where the beam is intercepted and shaped will give rise to an additional asymmetric focusing effect imposed upon the beam which may cause some spherical aberration and operate to defeat the purpose of intercepting the beam. In addition, secondary electrons will be emitted by the beam intercepting grid. These secondary electrons are directed toward the display screen by the electrostatic field in the vicinity of the beam intercepting grid causing loss of contrast and/or loss of purity in a color CRT. A third problem also arises from the energetic electrons incident upon the beam intercepting grid about a beam shaping aperture. Because the electrons are intercepted in a high voltage region of the electron gun and have a high kinetic energy (an electron gun typically has a focus voltage of a few thousand volts), the intercepted high energy electrons release their kinetic energy at the aperture region causing a substantial increase in the temperature of the beam intercepting grid which in some cases may become vaporized before this energy can be dissipated.
The present invention addresses the aforementioned limitations of the prior art by compensating for the asymmetric electrostatic field of a main lens in an electron gun and correcting for the resulting electron beam spherical aberration in a color CRT to provide a small, circular beam spot on the CRT's display screen. The present invention corrects the spherical aberration in an electron beam arising from a main lens asymmetric electrostatic focus field by providing a compensated electron beam cross sectional shape as the beam enters the main lens to provide improved electron beam spot performance. The present invention also may be used to shape an electron beam in a monochrome CRT to provide optimum display pixel density and/or to eliminate display discontinuities and provide a smooth video display.